Mordenkainen's Sword is a level 7 concentration spell for bards or wizards that costs an action to cast and summons a blade of force for one minute. It attacks for 3d10 force damage, and you can repeat the attack and move it for up to 20 feet using your bonus action on subsequent turns.
Spiritual Weapon is a level 2 spell for clerics that costs a bonus action to cast and summons a spectral weapon for one minute. You likewise can repeat the attack and move it for up to 20 feet using your bonus action on subsequent turns. The weapon deals d8+your spellcasting ability modifier force damage. It can be upcast, and if you upcast it to a level 6, it deals 3d8 + your ability modifier.
Most full casting builds aim to put their best stat on their spellcasting ability and start with a +3 bonus, so by the time they can cast level 7 spells they are maximized to +5. This means spiritual weapon by level six will deal an expected 18.5 force damage, vs Mordenkainen's Sword's 16.5.
Comparing the two spells side by side, Spiritual weapon seems just better -- everything else being equal, it does not require concentration, does not need a full action to cast, deals more damage, and you can cast it using lower level slots if you run out of high level slots. And it is level 6 upcast, not 7.
Is there something that explains or supports the higher spell level assigned to Mordenkainen's Sword, for example related to class balance? Or is the sword at the right level, and Spiritual Weapon is unusually strong and not a good spell to use as a benchmark? Or is the sword weak for what it does and would have better been given a lower level, if so, which one would be most appropriate?
